1. Field of the Invention
This invention relates to a method and apparatus for reducing multipath noise in an audio signal transmitted by frequency modulation of a carrier wave (an FM audio signal), and to an FM receiver including a multipath noise reducer.
2. Description of the Related Art
Radio receivers must contend with various types of electromagnetic noise. Radio broadcast receivers mounted in automobiles, for example, pick up ignition noise and mirror noise, which are impulsive in character and are generally referred to as impulse noise. Such car radios also experience episodes of multipath noise due to reflection of radio waves from hills, high buildings, and other passing objects. Multipath noise occurs because the car radio antenna receives both a line-of-sight signal, coming directly from the transmitting antenna, and reflected signals, reflected from the passing objects. The reflected signals tend to be out of phase with the line-of-sight signal, causing the line-of-sight signal to be partly attenuated by the reflected signals, so that the received signal level is reduced. The resulting deterioration in quality of the audio output from a car radio is a familiar experience to automobile riders.
Various methods of reducing noise are known. In an FM stereo car radio, one method is to detect the strength of the electric field received at the antenna, and take noise countermeasures when the field is weak. For example, it is possible to reduce the degree of stereo separation, or to switch completely from stereo to monaural operation, a countermeasure referred to below as stereo separation control. It is also possible to attenuate or xe2x80x9ccutxe2x80x9d high-frequency components in the demodulated signal, a countermeasure referred to below as high-cut control. Both of these countermeasures improve the signal-to-noise (S/N) ratio during intervals when the electric field received at the antenna is weak.
To reduce impulse noise, car radios may also include an impulse noise reducer that detects the onset of impulse noise and generates a gate signal having a predetermined length sufficient to cover the expected duration of the impulse noise, which is typically less than a millisecond. When the gate signal is active, the signal output by the car radio is held constant, effectively suppressing the noise. The gate pulse used in this type of impulse noise reducer, however, is too short to mask multipath noise, which usually lasts considerably longer than a millisecond.
Japanese Unexamined Patent Application Publication No. H2-283129 discloses a noise reducer having an additional multipath noise detector and generating longer gate pulses to cover multipath noise intervals, but these long gate pulse noticeably distort the audio output signal. Further details will be given in the detailed description of the invention.
An object of the present invention is to reduce multipath noise present in a demodulated signal obtained from a frequency-modulated signal without causing unnecessary distortion of the demodulated signal.
A more specific object of the invention is to reduce multipath spike noise in the demodulated signal without distorting the intervals between noise spikes.
In the invented method of reducing multipath noise, a high-frequency signal is extracted from the demodulated signal, and a noise reduction coefficient is generated from the extracted high-frequency signal. The demodulated signal is also separated into a high-frequency component and a low-frequency component, and the high-frequency component is multiplied by the noise reduction coefficient. The resulting product signal is added to the low-frequency component to obtain an output signal in which multipath noise is reduced.
In this way, the noise reduction coefficient is able to attenuate noise spikes without unnecessary attenuation of other parts of the demodulated signal.
The process of generating the noise reduction coefficient from the extracted high-frequency signal may include smoothing the extracted high-frequency signal, rectifying the extracted high-frequency signal by taking its absolute value, limiting the rectified high-frequency signal to a predetermined upper limit value, and offsetting parts of the rectified high-frequency signal that are less than a predetermined offset value by raising them to the offset value. All of these measures tend to reduce distortion of the output signal.
Further reduction of distortion can be obtained by comparing the extracted high-frequency signal with a threshold, thereby detecting noise spikes, replacing the demodulated signal with an interpolated signal during the noise spikes noise, and separating the low-frequency component from the resulting modified demodulated signal. The noise spikes are preferably detected from a rectified version of the extracted high-frequency signal, so that the noise spikes can be located accurately.
The invention also provides a multipath noise reducer that operates according to the method described above, and an FM radio receiver incorporating the invented multipath noise reducer.